Protesters have complained of an intensifying campaign of intimidation in a country where pro-Iranian armed groups integrated into the security forces wield growing influence.

The body of 49-year-old father of five Ali al-Lami was found overnight with gunshot wounds to the head, according to his friends, who said he had arrived in Baghdad just days earlier to join the protests.

"It was the militias of a corrupt government that killed him," said a close friend, Tayssir al-Atabi.

A police source said the attackers had used guns with silencers, while forensic experts said Lami had been struck by three bullets.

Iraq's capital and its Shiite-majority south have been gripped by more than two months of anti-government demonstrations in which more than 450 people have died and 25,000 have been wounded.

Despite the threats and violence, protesters massed in Baghdad and the south again on Wednesday calling for the "fall of the regime".

The UN urged the Iraqi authorities to hold to account the perpetrators of a string of murders and abductions of activists and protesters.

"Groups referred to as 'militia', 'unknown third parties', 'armed entities', 'outlaws' and 'spoilers' are responsible for the deliberate killings and abductions of demonstrators," said a UN report released Wednesday.

"These acts contribute to a climate of anger and fear. The government must identify those groups responsible without delay and hold perpetrators accountable."

- Near-daily disappearances -

The role of the Hashed al-Shaabi, a network of armed groups integrated into the state, has come under increased scrutiny.

Founded in 2014 to fight IS jihadists, the Hashed is made up of mostly Shiite factions, many of which have been backed by Iran.

It initially supported the government over protests but switched sides, although demonstrators fear Hashed fighters' presence at rallies could derail their anti-regime movement.

Following an attack against protesters in Baghdad at the weekend that left 24 dead, Hashed chief Faleh al-Fayyadh ordered his men to stay away from rallies, in what was seen by demonstrators as an admission of guilt.

Since October 1, demonstrators in Baghdad and southern cities have disappeared almost daily.

The authorities say they have been unable to identify the perpetrators.

In most cases, the protesters are taken from near their homes as they return from protests, leaving their relatives fearing the worst.

Lami had left his southern hometown of Kut days earlier to protest with his children in Baghdad, and had called on social media for protesters to rally peacefully.

He is the third protester to be killed since December 2.

Last week, the bruised body of a 19-year-old protester was dumped outside her home in Baghdad, and on Sunday a prominent Karbala activist was shot dead by a gunman who was riding on the back of a motorbike.